It's not illegal to have a monopoly. It's illegal to exploit it in certain ways.

This is mostly a distinction without a difference in practice. Exploit it in certain ways and your monopoly gets broken apart- thus illegal. See ATT/ Ma Bell

It's a very practical distinction since you can avoid exploiting it in certain ways and thus not get into any trouble at all. Violator monopolies do not exert predatory pricing, etc, by accident.

Very good point.